1. Field of the Invention
This invention relates to error correction in digital television signals, and more particularly to vertical parity correction.
2. Description of the Prior Art
In a digital video tape recorder (DVTR), an incoming television signal to be recorded is sampled, the samples are pulse code modulation coded into digital form, the resulting digital data signals are recorded on a magnetic tape and subsequently reproduced, the reproduced digital data signals are decoded, and the decoded signals are used to form an analog signal corresponding to the original television signal. If errors occur in the handling of the digital signals, for example due to noise or tape drop-out occurring in the DVTR, the digital signals are corrupted and then the reformed analog television signal does not correspond exactly to the original analog television signal, and a resulting television picture is degraded.
Such errors and their detrimental effects can be reduced by correction and concealment. Correction involves the production and use of additional data signals purely for the purposes of error detection and correction, these additional data signals otherwise being redundant. While correction provides good results, it cannot generally be used as the sole means of dealing with errors, because a comprehensive error correction capability would require an excessive amount of additional data which might overload the data handling paths or raise the data rate to an unacceptable level. Concealment comprises the replacement of corrupted data signals by data signals generated using available uncorrupted data signals. This method relies largely for accuracy on the strong correlation that exists in a television signal.
When a DVTR is reproducing in the normal reproducing mode, that is at the normal replay speed, the data is being recovered from the tape at the same overall rate that is required for display on the television receiver. It is only necessary to provide time-base correction to synchronize the data reproduced from the tape with stable output synchronizing signals. Increasingly, however, there is a requirement for reproduction at non-normal speeds. These are generally referred to as special reproduction modes and include still, slow motion, reverse motion and the so-called shuttle mode which comprises the whole range of speeds from fast forward to fast reverse and which is of particular utility in searching and editing.
Reproduction of a digital television signal at non-normal speeds implies some sort of temporary storage of the reproduced data prior to supply to a television receiver, and the present invention is based on the realisation that this temporary storage can additionally be used to effect improved vertical parity correction.